Methods of manufacturing board from raw materials which are based on lignocellulose are well-known and widely applied. These manufacturing processes generally comprise the following steps: disintegration of the raw material to particles and/or fibers of a suitable size, drying to a definite moisture ratio and gluing the material prior or, subsequent to the drying, forming the glued material into the form of a mat, which can be built up of several layers, possibly cold prepressing, preheating, surface nozzle-spraying in some instances, and hot pressing simultaneously with pressure and heat applied in either a discontinuous or a continuous press, to produce a finished board.
During conventional hot pressing, the pressed material is primarily heated by means of thermal conduction from the adjacent heating plates or steel belts which have a temperature of 150.degree. to 250.degree. C., depending on the type of product being pressed, the glue used, the desired capacity, and other such factors. The moisture of the material which is closest to the heat source is thus evaporated, whereby as the pressing continues a dry layer develops and a steam front successively moves from each side inward to the center of the board. The temperature in this developing layer rises to at least 100.degree. C., which causes normal glues to cure. When the steam front has arrived at the center of the board, the temperature there has risen to at least 100.degree. C. and the board begins to harden, even at the center at which point pressing can be terminated within a number of seconds. This applies to the use of conventional urea formaldehyde glue (UF) and similar glues, such as melamine-fortified glues (MUF). When other glues with higher curing temperatures are used, a higher temperature and a higher pressure must develop in the board before curing can take place. In connection with conventional hot pressing, methods have been developed to control the density profile of the board in the thickness direction. In most cases it is desired to achieve a high density in the surface layers in order to improve paintability, strength and the like, and a reasonably low density in the central layer, i.e., as low as possible for holding the board weight and cost down, but sufficiently high for achieving an acceptable internal bond strength and the like. During the manufacture of particle board, more finely disintegrated particles with a slightly higher moisture content in the surface layers often have been used, for example, in order to achieve a higher density in the surface layers of the board. During the manufacture of MDF (Medium Density Fiberboard), which have a homogeneous material structure, methods have been developed by utilization of a controlled distance between the heat sources in order to approach the final position successively in a predetermined manner as the steam front moves inward towards the center. See, for example, Swedish patent No. 469,270 for a continuous press, and Swedish pat. appln, No. 93 00772-2 for a single opening discontinuous press. These methods, which were developed for MDF, are now at least partly used for other types of board.
In order to achieve the desired density profile, a press must be capable of applying a high surface pressure at high temperatures. This in itself is no problem for a discontinuous press. However, such a press has other disadvantages, such as, for example, inferior thickness tolerances. For continuous presses the required high surface pressure and simultaneously high temperature have resulted in the need for expensive precision solutions for the roller table between the steel belt and the underlying heating plate. The method of supplying heat to the board by means of thermal conduction further entails heating which takes a relatively long time, which thus results in great press lengths (large press surfaces). Presses up to about 40 m in length have thus been delivered. Furthermore, with a continuous press it is practically impossible to make the heating plates of the press sufficiently flexible and, therefore, the density profile cannot be formed with as great a freedom as in the case of discontinuous pressing.
In addition, the continuous presses utilized to-day are restricted as regards temperature (because of the lubricating oil in the roller table), which means that not all types of board can be pressed.
Another method of board manufacture, which is based on the supply of steam in between the heating plates in a discontinuous press, has also been found to have limited use. The material in that case is heated within seconds of the supply of steam and, therefore, the heating time can be radically shortened. Moreover, after the steam has been supplied the resistance of the material against compression reduces considerably. This is a positive feature, thus implying that the press could be designed with less press power, and with a much shorter length (smaller press surface). In order to achieve a desired density profile of a boards manufactured according to this method, however, it is necessary for conventional pressing techniques with a high surface pressure and thermal conduction from conventional heating plates at the beginning of the press cycle to be applied, whereby a surface layer with a high density can be obtained after a long heating period. Thereafter, steam can be injected for heating the central part of the board. This has given rise to problems, because steam has to be blown through the newly formed surface layer with a high density, and because the pressing time during the period of high pressures and thermal conduction has been extended considerably. As a consequence, a steam press operating according to this concept has a much lower capacity, a larger press surface, and requires a high press power than would be required if a uniform density had been attained.
During all of the manufacturing methods referred to above, a soft surface layer is obtained, which has lower strength, unacceptable paintability, etc., which implies that this layer must be removed, such as by grinding. The resulting material loss is 5-15%, depending on the type of board, its thickness, etc.
One object of the present invention is to provide a method of continuous pressing of board of lignocellulosic material, which renders it possible to make use of the advantages of steam heating, i.e., so that the equipment can be designed with considerably smaller press surface and with lower press power, i.e., less expensive, and preferably without heating plates, whereby the present precision solutions with roller tables can be eliminated, thus rendering the equipment even less expensive, and yet having the possibility of achieving desired density profiles.
Another object of the present invention is to make the manufacturing process so flexible that different density profiles and surface properties can be formed in new ways and thereby new fields of application for board can be created.